Self-organized vegetation patterning as a fingerprint of climate and human impact on semi-arid ecosystems

Spatially periodic vegetation patterns are well known in arid and semi-arid regions around the world. Mathematical models have been developed that attribute this phenomenon to a symmetry-breaking instability. Such models are based on the interplay between competitive and facilitative influences that the vegetation exerts on its own dynamics when it is constrained by arid conditions, but evidence for these predictions is still lacking. Moreover, not all models can account for the development of regularly spaced spots of bare ground in the absence of a soil prepattern. We applied Fourier analysis to high-resolution, remotely sensed data taken at either end of a 40-year interval in southern Niger. Statistical comparisons based on this textural characterization gave us broad-scale evidence that the decrease in rainfall over recent decades in the sub-Saharan Sahel has been accompanied by a detectable shift from homogeneous vegetation cover to spotted patterns marked by a spatial frequency of about 20 cycles km-1. Wood cutting and grazing by domestic animals have led to a much more marked transition in unprotected areas than in a protected reserve. Field measurements demonstrated that the dominant spatial frequency was endogenous rather than reflecting the spatial variation of any pre-existing heterogeneity in soil properties. All these results support the use of models that can account for periodic vegetation patterns without invoking substrate heterogeneity or anisotropy, and provide new elements for further developments, refinements and tests. This study underlines the potential of studying vegetation pattern properties for monitoring climatic and human impacts on the extensive fragile areas bordering hot deserts. Explicit consideration of vegetation self-patterning may also improve our understanding of vegetation and climate interactions in arid areas. © 2006 The Authors.

Acquired epidermodysplasia verruciformis syndrome in HIV-infected pediatric patients: Prospective treatment trial with topical glycolic acid and human papillomavirus genotype characterization

SCOPUS: ar.j

IGF-1 or insulin, and the TSH cyclic AMP cascade separately control dog and human thyroid cell growth and DNA synthesis, and complement each other in inducing mitogenesis.

The regular doubling of cell mass, and therefore of cell protein content, is required for repetitive cell divisions. Preliminary observations have shown that in dog thyrocytes insulin induces protein accumulation but not DNA synthesis, while TSH does not increase protein accumulation but triggers DNA synthesis in the presence of insulin. We show here that EGF and phorbol myristate ester complement insulin action in the same way. HGF is the only factor activating both protein accumulation and DNA synthesis. The effects of insulin on protein accumulation and in permitting the TSH effect are reproduced by IGF-1 and are mediated, at least in part by the IGF-1 receptor. The concentration effect curves are similar for both effects. Similar results are obtained in human thyrocytes. They reflect true cell growth, as shown by increases in RNA content and cell size. Carbachol and fetal calf serum also stimulate protein synthesis and accumulation without triggering DNA synthesis, but they are not permissive for the mitogenic effects of TSH or of the general adenylate cyclase activator, forskolin. Moreover the mitogenic effect of TSH greatly decreased in cells deprived of insulin for 2 days although these cells remain hypertrophic. Hypertrophy may therefore be necessary for cell division, but it is not sufficient to permit it. Three different mechanisms can therefore be distinguished in the mitogenic action of TSH: (1) the increase of cell mass (hypertrophy) induced by insulin or IGF-1; (2) the permissive effect of insulin or IGF-1 on the mitogenic effect of TSH which may involve both the increase of cell mass and the induction of specific proteins such as cyclin D3 and (3) the mitogenic effect of the TSH cyclic AMP cascade proper.

Environmental and Human Pathogenic Microorganisms

As the study of interactions between pathogenic microorganisms and their environment is part of microbial ecology, this chapter reviews the different types of human pathogens found in the environment, the different types of fecal indicators used in water quality monitoring, the biotic and abiotic factors affecting the survival and the infectivity of pathogenic microorganisms during their transportation in the environment, and the methods presently available to detect rare microorganisms in environmental samples. This chapter exclusively focuses on human pathogens.

Atmospheric dispersion, environmental effects and potential health hazard associated with the low-altitude gas plume of Masaya volcano, Nicaragua

info:eu-repo/semantics/published